The invention relates to a device used in the performance of MRS (magnetic resonance spectroscopy) and MRI (magnetic resonance imaging) experiments on small animals. In particular, the invention relates to a device and a method for positioning the small animal simply and precisely with respect to the radio-frequency (RF) antenna, an ‘antenna’ being a device for transmitting and receiving electromagnetic waves.
When a patient or an animal, hereinafter always referred to as “test object,” is placed in a strong magnetic field, the tissue of the test object is magnetized due to the behavior of certain atomic nuclei. This nuclear magnetic field has a component that rotates at a specific frequency. This phenomenon is known as nuclear magnetic resonance (NMR). The resonance frequency is proportional to the strength of the strong magnetic field and is termed the Larmor frequency. Practically all clinical applications of magnetic resonance (MR) are based on the manipulation of the nuclear magnetic field of hydrogen, which at a field strength of 1.5 Tesla has a Larmor frequency of 63.87 MHz. The MR image is a representation of the NMR signal intensity. To provide a useful image, different tissue types must produce different signal intensities (contrast). For this, the MRI scanner must firstly generate detectable NMR signals in the tissue, and secondly, be able to resolve them spatially.
MRI systems are widely used medical and diagnostic apparatus. The primary components of an MRI system are the magnet, which generates a stable and very strong magnetic field (B0), the gradient coils, which generate an additional variable magnetic field, and the RF transmitter antenna, which is used to transmit energy into the test object and encode the spatial position. RF reception antennas are used to receive the NMR signal from the test object. It is possible to use an RF antenna both as a reception and as a transmission antenna. A computer controls the entire procedure and is required to process the information received.
A Region of Interest (ROI) will hereafter be used to designate an area of the test object that is of particular significance. This might be, for example, an organ or a body part of a small animal to be examined.
Let the Field of View (FOV) define the size of the region within which a sufficiently strong signal is generated by the RF antenna. Anything that lies outside the FOV of an RF antenna cannot be represented in the MR image.
The Signal to Noise Ratio (SNR) is a criterion for the image quality in magnetic resonance tomography. The SNR is the ratio between the effective NMR signal acquired by the MRI system and the random noise signals acquired by the MRI system.
The RF antenna systems of MRI devices fall into two categories. One category contains antenna systems that use the same RF antenna to send and to transmit, termed transmit-receive antennas.
The second category contains antenna systems that use separate RF antennas to send and to transmit RF signals, one RF antenna to transmit RF signals, termed transmit antenna, and one RF antenna to receive NMR signals, termed receive antenna.
In the case of MR examinations of the organs or body parts of small animals, RF antennas that are adapted to the geometry of the ROI are increasingly being used to receive nuclear magnetic resonance signals (MR signals). These RF antennas are positioned as closely as possible to the body surface directly at the organ or body part of the small animal being examined. In contrast to RF antenna that are positioned at a greater distance from the small animal, which are usually used to produce a cross-section of the entire small animal, RF antennas of this type are considerably smaller in their geometrical dimensions. This reduces the noise component caused by the dielectric losses inside the body of the small animal, which means that the SNR of an RF antenna adapted to the geometry of the ROI is, in principle, better than that of a more remote antenna. The disadvantage, however, is that a smaller RF antenna is only able to generate an image within a limited spatial extent, which approximately corresponds to the typical dimensions of the RF antenna.
The RF antennas described above, which are adapted to the geometry of the ROI, are frequently surface coils or so-called phased array coils. The information given above, however, applies nonetheless also to coil types such as birdcage, solenoid, Helmholtz coils, and strip line antennas.
A surface coil is understood to be a coil of conducting material that functions as an RF antenna and that is in direct contact with the test object.
A phased array coil is an arrangement of several RF antennas that are operated in parallel.
Birdcage coils are known, for example, from U.S. Pat. No. 4,680,548. Their two ladder loops usually have the shape of two equally sized coaxial circles that are connected to each other by the ladder. This structure lends the MR coil arrangement the appearance of a birdcage, which is why the coil arrangement is commonly referred to as a “birdcage coil” among specialists.
Reducing the size of the RF antenna results in one essential problem:
Because the FOV of a smaller RF antenna is smaller in accordance with the dimensions of the RF antenna, it is correspondingly more difficult to position the small animal with respect to the RF antenna such that the organ or body part to be examined (more general: the ROI) is located inside the FOV of the RF antenna. In particular, it is often necessary to position the RF antenna as close as possible to the small animal, as the intensity of the RF field decreases much more quickly in the case of smaller RF antennas. A rule of thumb states that the FOV of a surface coil extends a half coil diameter into the test object.
As a result, the positioning of small animals with respect to the RF antenna is becoming an increasingly important aspect.
Frequently, precise positioning of the ROI with respect to the RF antenna outside the magnet is not possible because the position of the ROI cannot be determined precisely enough without MR imaging. For example, if, in MR imaging of the region of the heart of small animals, the position of the heart is determined by feeling, this results in a positioning error of approx. ±5 mm.
In prior art, this problem of positioning the small animal with respect to the RF antenna is essentially solved by two different means:
Type A:
FIG. 6a shows a typical device used in the prior art to position a small animal 3 with respect to an RF antenna. The small animal 3 is located in a cradle 5. The small animal 3 is immobilized in this cradle 5 either with a stereotactic fixing aid or with other aids, or it is positioned in cradle 5 without being immobilized. Essentially, however, the small animal 3 is moved together with the cradle 5. To position the RF antenna with respect to the small animal 3, the former is attached to cradle 5 by a particular method. The small animal 3 with cradle 5 and the RF antenna positioned with respect to it are now moved into the MRI magnet as a single unit. It is typical of this type of device that the relative position between the small animal 3 and the HF antenna cannot be changed once the device has moved inside the magnet.
U.S. Pat. No. 6,275,723 B1 describes a device for stereotactic immobilization of a small animal for MRI. It describes how this device can be positioned in the magnet and how an RF antenna can additionally be fixed on the device.
US 2001 053 878 A1 describes an immobilization facility for neuroimaging of animals in MRI systems, wherein the animal is fixed in a cradle together with a holder for the head of the animal. It is described how the holder for the head of the animal can contain an RF antenna.
The problem of conventional devices of Type A is that the relative position between the small animal and the RF antenna cannot be changed once the device is located inside the magnet. If the MR image shows that the RF antenna is not correctly positioned with respect to the animal because, for example, the position of the heart was not correctly felt, the device must be moved out of the magnet and the animal repositioned with respect to the RF antenna outside the magnet.
Type: B
FIG. 6b shows a second typical device, which is the prior art for positioning a small animal 3 with respect to an RF antenna. The small animal 3 is located in a cradle 5. The small animal 3 is immobilized in this cradle 5 either with a stereotactic fixing aid or with other aids, or it is positioned in cradle 5 without being immobilized. Essentially, however, the small animal 3 is moved together with the cradle 5, wherein the RF antenna is not permanently fixed to cradle 5 but is located inside the MRI magnet. To position the RF antenna with respect to small animal 3, the cradle 5 together with the small animal 3 is introduced into the MRI magnet 7 and the RF antenna. The relative position of the small animal 3 with respect to the RF antenna can be changed inside the magnet by moving the cradle 5. It is typical of this type of device that the small animal and the RF antenna cannot be positioned relative to each other outside the magnet.
US 2005 027 190 A1 describes a system that simplifies the imaging of a test object in an MRI system. The system comprises, among other components, supports that simplify mounting of the RF antenna in the magnet. The publication also describes a device for the radial and axial positioning of a small animal holder with respect to the RF antenna. However, no provision is made for the preparation of the animal together with the RF antenna outside the magnet.
WO 94 28 431 A1 describes a device for radial and axial positioning of a small animal holder and of an RF antenna with respect to the magnet system without provision for moving the animal into the magnet together with the RF antenna.
Conventional devices of Type B pose the problem that the relative positioning of the test object with respect to the RF antenna outside the magnet cannot be monitored. But in work with live animals or when the small animal must be positioned as close to the RF antenna as possible, the possibility of monitoring this positioning outside the magnet is of great importance. If positioning can only be performed inside the magnet and cannot thus be verified visually, there is a danger that the small animal may be injured.
The object of this invention is to present a device, with which it may be possible to retrofit an existing tomography apparatus, for the relative positioning of a small animal with respect to an RF antenna for an MRI measurement, wherein this positioning can be effected both inside and outside the MRI magnet, with easy handling and without great additional technical effort.